1. Field of the Invention
The present invention relates to surface passivation of semiconductor devices and, more particularly, to metal-insulator-semiconductor type semiconductor devices and a method for production thereof, the metal layers of such devices protecting the semiconductor device from contamination and stabilizing the properties of the semiconductor devices.
2. Description of the Prior Art
Recently, there has been a demand that semiconductor devices, for example, transistors and integrated circuit devices, be miniaturized and integrated and that the stability and reliability of the semiconductor devices be improved, simultaneously.
It is known by persons having ordinary skill in the art that semiconductor circuit elements of semiconductor devices are contaminated by cations of alkali ions, especially, sodium ions (Na.sup.+), so that the stability of the semiconductor devices is poor. Accordingly, in order to decrease the effect of such contamination, it has been suggested to remove or to trap the contaminant in the steps of thermal-oxidizing a semiconductor substrate and of forming an electrode.
Namely, when the semiconductor substrate is oxidized, chlorine gas (Cl.sub.2) is introduced into an oxidizing atmosphere (cf. Kriegler, R. J., Cheng, Y. C. and Colton, D. R., "The Effect of HCl and Cl.sub.2 on the Thermal Oxidation of Silicon," Jour. of Electrochem. Soc., vol. 119, pp. 388-392, 1972). Thus, chlorine is contained in an oxide layer formed on the surface of the semiconductor substrate. The chlorine existing in such oxide layer electrically neutralizes the contaminant, especially sodium ions (Na.sup.+), entering the oxide layer. Furthermore, coating layers, such as an insulating layer lying under a conductive metal layer and on a surface of a semiconductor substrate, an insulating layer lying between conductive metal layers and a surface protecting layer covering the conductive metal layers, are made of phosphosilicate glass (PSG) or silicon nitride (Si.sub.3 N.sub.4) (cf. Schnable, G. L., Kern, W. and Comizzoli, R. B., "Passivation Coatings on Silicon Devices," Jour. of Electrochem. Soc., vol. 122, pp. 1092-1103, 1975). Such coating layers stop the contaminant, such as sodium ions (Na.sup.+), from passing through the coating layers. Especially, it is possible to trap sodium ions (Na.sup.+) existing in the oxide layer formed on the semiconductor substate by forming the phosphosilicate glass layer on the oxide layer.
However, the above-mentioned anticontamination treatments, i.e. so-called passivation treatments, are complex and are not always sufficiently effective. Although such passivation treatments are carried out, there is a definite danger that the semiconductor device will lose the stability of semiconductor circuit elements thereof due to adhesive contaminant and vapor on the surface of the semiconductor device. Such contaminant and vapor are contained in the atmosphere, and the semiconductor device is usually exposed to the atmosphere until the semiconductor device is gas-tightly enclosed in a package.
Accordingly, in order to decrease the above-mentioned danger, persons with ordinary skill in the art can resort to one of the following three methods, taking the above-mentioned passivation treatments into consideration.
According to one of the methods, during the thermal oxidation of the semiconductor substrate the quantity of chlorine gas introduced into the oxidizing atmosphere is increased to increase the chlorine contained in the formed oxide layer. However, this method has defects in that the surface of the semiconductor substrate is etched too strongly and becomes rough and the surface state density is increased at the oxide layer-semiconductor substrate interface.
According to another one of the methods, the phosphorus concentration in the phosphosilicate glass, i.e. in the surface protecting layer and/or the insulating layer lying between the conductive metal layers, is increased. However, this method has defects in that the hygroscopic property of the phosphosilicate glass increases and an ionic polarization in the glass is easily generated.
According to another one of the methods, the surface protecting layer or the insulating layer lying between the conductive metal layers is made of a multilayer which is comprised of two or more layers, for example, a phosphosilicate glass or silicon nitride-silicon dioxide double layer. However, this method has a defect in that, since the thickness of the protecting and/or insulating layers is substantially increased, such layers cannot be used in a large-scale integrated circuit semiconductor device which requires a minute circuit design. Especially, in a semiconductor device including a metal-insulator-semiconductor (MIS) type of a semiconductor circuit element, a certain thickness in the range of, e.g., from 100 A to 500 A, of the insulating layer is required, but it is very difficult to make such an insulating layer out of the above-mentioned multilayer.